Whither High Energy Lasers?

Maj Gen Donald L. Lamberson, PhD, USAF, Retired
Col Edward Duff, USAF, Retired
Don Washburn, PhD
Lt Col Courtney Holmberg, PhD, USAF*

Imagine an ability to execute speed-of-light attacks against enemy forces with massive bursts of photon energy, literally incinerating the intended target. The desire for such a weapon is not the exclusive result of an appetite nourished by Hollywood science fiction and action-movie cultures, nor is it even a recent phenomenon. Mankind has been intrigued by the concept of directing light against a target for a very long time, and the absence of today’s advanced technology did not preclude dreams and fantasies of novel weapons. An incident in “ancient” history illustrates this well.

The story takes place in the coastal city of Syracuse on the southeastern shore of Sicily, an island located across the Messina Straits from the southwestern coast of Italy. During the year 213 bc—2,217 years ago—Syracuse was the home of Archimedes. He was in his 75th year and after spending many years in Greece had returned to Syracuse for his retirement. Marcus Claudius Marcellus, the Roman commander, began attacking Syracuse during the second Punic War with a fleet of over 50 quinqueremes, vessels that were propelled by five banks of oars and filled with soldiers armed with all kinds of devices to overcome the city walls. Hiero, king of Syracuse, asked Archimedes to design a defense for the city. Attack after attack was successfully repelled, largely through the use of the mechanical engines engineered by Archimedes to hurl stones and other objects against the attackers. Marcellus demanded surrender; otherwise he promised to burn the entire city and execute all the people—Roman style. Fortunately for Syracuse, Archimedes had a secret weapon up his sleeve.

The geographical location of Syracuse led Marcellus to attack by sea from the east. He also chose to attack at daybreak so the sun would be at his back and in the eyes of the Syracuse defenders, hindering them from detecting and tracking his fleet. However, this geographical orientation also proved advantageous to Archimedes since the fleet’s approach would be at a well-defined, small angle from the position of the sun. Archimedes conceived of a defense that employed mirrors to reflect and focus the sunlight on the Roman ships as they approached the island. The energy’s flux—reflected and focused sunlight—was sufficient to set the ships’ tarred-fir planks on fire. In their first recorded use, relay mirrors destroyed Marcellus’s fleet.

Whether this story is real or just a fable will probably never be known.¹ However, it is known that although Syracuse—through Archimedes—won the battle, it soon lost the war. Marcellus landed his regrouped forces on the undefended western end of the island and captured Syracuse by land attack, unintentionally killing Archimedes in the process.

Nothing in the laws of physics would have precluded Archimedes from building and employing this extraordinary defensive weapon. Several experiments, some recent, have successfully demonstrated that even crude mirrors could concentrate sufficient energy to cause the storied effect. One experiment fitted an aiming device to bronze shields, traditional equipment for the soldiers of Syracuse, and was able to successfully focus the reflected sun’s energy and set wood on fire at several hundred meters. However, what really matters is not whether the story is true, but rather that it has persisted for over 2,000 years, which demonstrates the attractiveness and importance of such a capability.

Military research into high-energy lasers for weapons applications traces its beginnings to the early 1960s. Since then, significant advances in laser- power production, target tracking, and beam control have been made.² Systems such as the Airborne Laser Laboratory (ALL) of the early 1980s demonstrated that laser systems staged on airborne platforms could destroy enemy missiles. Nevertheless, are we any closer to fielding that revolutionary new capability for the war fighter?

The answer is “perhaps so.” How? When? The remainder of this article addresses the difficulty of transitioning new technologies to the war fighter and the importance of robust technology demonstrations for high-energy laser weapons systems. Additionally, it highlights two critical new-technology areas that will likely be the key to our long-term laser war-fighting capability.

Technology Transition

Consider the general problem of transitioning a new technology into war-fighting systems. The laser was invented in 1961, and high-energy devices were demonstrated several years later. The high-energy laser community is often criticized because there is not yet any production associated with high-energy laser weapons. It turns out that an extended period of incubation is true of most revolutionary technologies. Today we are all familiar with the rapid evolution of fielded computer technology and capability, but the transistor, which makes it all possible, was invented in 1940. Arguably, it was the 1980s before the accelerated development of computer technology really matured. The Air Force Scientific Advisory Board, under Dr. Gene McCall, characterized this phenomenon in its New World Vistas study of 1995.³

Consider a new technology which doubles in some attribute, say “relative importance,” every four years as shown in figure 1. It is interesting to see how the “relative importance” compounds over 40 years—three orders of magnitude (from 1 to 1,000) on the chart, with most of the acceleration being in the last few years. In fact, the first few years are barely distinguishable. This simple, nonlinear behavior seems to be a characteristic of many technologies; the computing world knows it well as Moore’s Law. An example in the weapons field is the development and fielding of precision strike weapons. The first prototypes of these now-so-familiar weapons were available in the early 1960s, but maturation did not occur until after Desert Storm in the early 1990s.

Where would we place today’s high-energy lasers on the chart of the hypothetical case just described? They are arguably somewhere around the knee of the curve, perhaps near the 30-year point. It is also plausible that the development of high-energy laser technology has nearly reached a point that, within the next decade, will enable most of the currently envisioned applications. Based on the simple exponential-growth analogy above, high-energy laser weapons are poised for a revolutionary leap. Whether or not US military interest in high-energy lasers actually accelerates these developments depends on many factors, not the least of which is the unfolding world situation and the demands and circumstances of the military operator. Although the research, development, and acquisition communities can neither predict nor control that requirement, the current trends of what the war fighter needs seem well entrenched—increased precision, rapid response, and tailored lethality to minimize collateral damage and reduce civilian casualties. What these communities can control and optimize is their readiness to transition high-energy laser weapons into production. The path to success is through mature and meaningful demonstrations on ground (mobile and fixed), naval, and airborne (tactical and strategic) platforms of interest.

Our recent experience is that today’s US military is quite imaginative and creative in applying the weapons it has on hand, even the so-called hi-tech weapons. In fact, it is common for the military operator to find applications for weapons that the weapon developer never had in mind. However, the typical operator is neither well informed nor overly interested in revolutionary technologies that may satisfy his requirements. That is why technology demonstrations, or early system prototypes, are so crucial in capturing the interest and imagination of the operator. It is important that these demonstrators show not only that the physics, engineering technology, and integration issues are understood, but also that they provide operators with sufficient access to demonstrators to whet their appetite for a new operational vision. If that is marketing, so be it. Demonstrator systems of this class tend to be very complex and expensive; they take years to develop. After all, depositing significant energy with centimeter-like precision at ranges from a few kilometers to perhaps thousands of kilometers is a very complex task. There are, however, three such high-energy laser demonstrators in the works today.

Technology Demonstrators

The Army’s Tactical High Energy Laser (THEL) (and its mobile variant), United States Special Operations Command’s (SOCOM) Advanced Tactical Laser (ATL), and the Missile Defense Agency’s (MDA) Airborne Laser (ABL) are the Department of Defense’s (DOD) major laser-weapon-technology demonstrators.

The high-energy laser community owes much to the Army’s successful demonstration of the THEL. The megawatt-class deuterium fluoride chemical laser, which lies at the heart of the THEL, is a very mature technology. It successfully engaged and destroyed several in-flight Katyusha rockets at ranges of several kilometers. The THEL’s successful demonstration has attracted the attention of many who are interested in tactical laser weapons. As a spin-off of the original effort, there is now a program to develop a mobile THEL (MTHEL).

The ATL is the latest demonstrator to be defined and programmed. The ATL uses a closed-cycle, chemical oxygen-iodine laser (COIL) with an appropriate beam control. The closed-cycle system captures all of the waste by-products, making it suitable for tactical employment. The ATL will be installed in a C-130 aircraft to demonstrate its ability to engage tactical targets from a moving platform at ranges of approximately 10 kilometers. This SOCOM demonstration program is important and should be completed in the next three to five years.

The MDA’s ABL program is the largest and most complex of the major high-energy laser demonstrators that the Air Force is executing. The ABL uses a very large COIL, and its components are integrated into the fuselage and systems of a Boeing 747-400 aircraft. It is designed to operate at very high altitudes (~40,000 ft) and have a capability to kill theater ballistic missiles (TBM) while they are still in their boost phase. The aircraft has been modified and flown to Edwards AFB, California, where its laser modules are being tested in a dedicated test cell. Its beam-control system is finishing a low-power checkout in Sunnyvale, California, before it is installed in the aircraft for low-power flight tests. Finally, the high-energy COIL will be installed on the aircraft for full system integration and testing. These efforts should be completed in about two years.

To propagate and focus laser energy over a distance of hundreds of kilometers and through the atmospheric turbulence that exists above 40,000 feet requires a robust atmospheric compensation or correction system for the laser beam. An adaptive-optics technology system has been built and shown to achieve good results at low power, but not yet at high power. The ABL, therefore, remains the major demonstrator by which to judge the maturity of US high-energy laser-engineering knowledge.

The success of these demonstrators will directly affect not only the transition of laser weapons into production but also the prospect of more advanced applications. One can anticipate a “window of opportunity” to open with the success of the ABL, ATL, and MTHEL, which will define high-energy laser-weapon activities for some time to come.

New Technology Highlights

This article limits itself to discussing electric solid-state lasers and relay mirrors. These topics, because of their extraordinary importance, deserve emphasis before all other high-energy laser-technology research.

Before this article launches into the benefits of solid-state lasers, it is useful to understand the technological maturity of high-energy lasers in general—both chemical and electric solid-state. Researchers know very well how to generate megawatts (MW) of laser energy with the gases from chemical lasers. With chemical oxygen-iodine and deuterium-fluoride lasers, we can also get suitably high-quality beams at full power. It is no accident that all three of the demonstrators discussed previously use chemical lasers. Currently that is the only way to achieve an excess of 10 kilowatts (kW) of average power, which is required for the desired effects on targets. So it is reasonable to assume that chemical lasers will be the engines of choice for large, strategic, high-energy laser applications requiring laser powers on the order of a megawatt (MW or 103 kW). Numerous studies have been conducted that suggest tactical applications of high-energy lasers become relevant at around the 100 kW level.⁴ For power levels in this range, solid-state lasers have clear advantages when compared to chemical lasers, which include being able to avoid the difficult challenge of providing and disposing of hazardous chemicals during battlefield operations. Neither is it clear that chemical gas lasers can be efficiently packaged for the small volumes associated with trucks or fighter aircraft.

Although significant challenges exist, electric solid-state lasers have great potential and are very attractive for tactical applications. Since we are already accustomed to generating and distributing electrical power on our platforms to run various subsystems, the logistics of electric solid-state lasers appears much more simple and attractive than that for chemical lasers. An “unlimited magazine,” where a laser-weapon platform has “laser bullets” as long as it has fuel, is also very appealing. Likewise, it appears that solid-state lasers can be more efficiently packaged. Currently, the maximum power achieved by solid-state lasers is around 15 kW with relatively poor beam quality. Scaling these lasers to attain a system with higher power and high brightness is a significant challenge. A major joint effort is under way to demonstrate the nation’s first weapon-class tactical laser small enough to fit on board combat aircraft, ground vehicles, and naval vessels.

The DOD High Energy Laser Joint Technology Office in Albuquerque, New Mexico, is leading the Joint High-Power Solid-State Laser Development Program with significant participation from industry and each of the services. The Program Research and Development Announcement (PRDA), dated August 2002, stated that the goal of the program is to demonstrate and deliver a 25 kW–class solid-state laser by December 2004. The PRDA also emphasized system characteristics such as beam quality, size, weight, efficiency, reliability, and ruggedness as key factors in establishing a scalable design path for a 100 kW system capable of integration onto tactical platforms. These goals are challenging and even more so on the prescribed schedule. Raytheon Company in El Segundo, California, and Northrop Grumman Space Technology in Redondo Beach, California, each won contract awards to pursue separate and distinct approaches. Additionally, the DOD selected Lawrence Livermore National Lab’s solid-state heat-capacity laser program, sponsored by the Army, to join in the competition.

Once these demonstrations establish the feasibility of high-energy laser weapons, the next question becomes, How can their range be extended? Atmospheric absorption, atmospheric turbulence, and curvature of the earth all limit the full potential of high-energy laser weapons. To compensate for these limitations, developers could build larger, more powerful lasers; use larger primary telescope optics; or attempt to place the laser system on high-flying platforms or even in space. For a given platform, however, the volume and weight constraints are likely to be very limiting. Researchers should continue to increase the brightness of the laser beam through more advanced beam-control techniques, and not solely for increased range. Nevertheless, this technique also is limited. As the performance of high-energy laser systems continues to improve on the margins, relay-mirror configuration could be another range-boosting option to consider.

Relays are not a new idea—Archimedes used a relay optic with solar power as a weapon. The Strategic Defense Initiative Office also considered relays in the early days of missile defense. However, at that time, the poor laser-beam quality that researchers were able to transmit to the relay system was a critical limitation. Researchers at the Starfire Optical Range, a division of the Air Force Research Laboratory’s (AFRL) Directed Energy Directorate at Kirtland AFB, New Mexico, have been diligently working that issue for some time and now know how to solve that problem. Scientists use a cooperative beacon and adaptive optics in the source-to-relay uplink to sense and then minimize atmospheric aberration. Therefore, a bifocal relay mirror effectively puts the laser source at the mirror. This dramatically increases system brightness and intensity on the target at a constant range or extends the laser’s range to a target while retaining the original levels of brightness and intensity. The implications of this are easy to understand.

The separation of the laser source from the beam-directing system allows each subsystem to operate in its most advantageous environment. In addition to the substantial range extension, technologists are just now beginning to understand the system flexibility such separation allows. The heavy, high-energy, and illuminator laser sources can be kept on the surface—ground- or sea-based platforms—far removed from the actual fighting, easing maintenance and allowing for the generation of beams with higher power. The optical relay system will be above most of the atmosphere, minimizing the adverse effects caused by atmospheric turbulence. Additionally, a single laser using a network of multiple mirrors could overcome horizon limitations and generate alternate line-of-sight paths to attack targets occluded by clouds or other obstacles. Some relay-platform concepts include networking multiple lasers for a single relay that can be pushed forward in the battlespace and occupy the high ground, essentially loitering in a geostationary position high above an area of interest.

When one previously considered the advantages and disadvantages of relay-mirror systems on various host platforms—manned aircraft, unmanned air vehicles, and even high-altitude airships—satellite relays were likely to have been seen as the most advantageous. However, the advent of MDA’s High-Altitude Airship (HAA) program now allows one to consider additional trade-offs when deciding to locate a relay-mirror system in space or on an airship. Relays mounted on a space-based platform offer the advantage of a large coverage area. That coverage comes with a high price tag—the cost of a large number of satellites for persistent offensive and defensive global coverage or a lesser cost associated with fewer satellites and an offensive capability only. Cost is probably the biggest disadvantage to a space-based system—both for an operational capability as well as for its demonstration. In contrast, an airship-based system would be limited to regional coverage, have fewer vehicles deployed, and, according to planners’ current estimates, a modest cost for both an operational and demonstration capability.

While support is growing for demonstrating relay mirrors on an HAA, other efforts are also under way. The services recently held a Relay Mirror Workshop at Kirtland AFB and studied ways to develop and demonstrate its technology. The AFRL’s Directed Energy Directorate calls its overall relay-mirror paradigm Evolutionary Air and Space Global Laser Engagement, or EAGLE (see fig. 2 and table 1). They have several experiments planned in the near term for potential relay-mirror concepts used in conjunction with ground-based lasers.

Table 1. Future missions of a relay-mirror system

Missions
Battlespace preparation
Target designation
Air/ground attack
Space control
    - Antisatellite (ASAT)
    - Defensive satellite (DSAT)
Asset protection
Cruise missile defense
Ballistic missile defense
    - Active tracking
    - Discrimination
    - Theater missile defense (TMD)

While the advantages of a relay-mirror system are many, so are the technical challenges. First, engineers must control the beam characteristics of both the illuminator and the high-energy laser to minimize the size of the mirror that receives the high-power beam. Likewise, engineers must create an uplink capability that can acquire and actively track the location of the relay mirror as well as provide the information required for the adaptive-optics feedback loops. Although conventional adaptive optics can accomplish many useful missions, incorporating advanced adaptive optics into the source and relay systems will increase deployment opportunities by maximizing the system’s range and efficiency.

Engineers face a different set of technical challenges in developing the relay platform (see fig. 3 and table 2). Despite these significant challenges, relay mirrors are a truly transformational technology for high-energy laser weapons systems.

Table 2. Technical challenges of a relay-mirror system

Source
Illuminator characteristics
HEL characteristics
Thermal management
Power and energy

Uplink (source link)
Acquire and actively track relay mirror
Adaptive optics on relay-mirror beacon
Propagate outgoing beam
Advanced adaptive optics
Thermal blooming

Relay platform
Dual line-of-sight stabilization
Acquisition, tracking, and pointing (ATP) in two directions
Boresight alignment and beam cleanup
Rapid retargeting
Separation uplink beams for downlink point-ahead
Lightweight optics
Lightweight illuminator/designator
Internal throughput (coatings, etc.)
Thermal management
Integrated power, energy, and thermal management

Target link
Passive tracking and pointing
Active tracking and pointing
Adaptive optics wave-front sensor (WFS) of target beacon
Aim-point maintenance
Thermal blooming
Tracking through clutter

Target
Active track interactions
Lethality requirements
Target interaction

Relay mirrors not only will enable lethal capabilities, but also will provide numerous opportunities for low-power laser-sensor applications. The 1999 AFRL “Lasers and Space Optical Systems Study” extensively reviewed these applications and concluded that relay mirrors could handle various irradiance, as well as different wavelengths, which could enable a number of long-range sensing applications. The same arguments for achieving system flexibility apply to these applications in the same way as they do for high-power applications.

Conclusions

Clearly there are major milestones, significant challenges, and exciting new technologies facing the high-energy laser community for the next decade or so. High-energy laser weapons research is arguably on the knee of the technological development curve. Weapon-class chemical laser systems—THEL, ATL, and ABL—have demonstrated, or will soon do so, their worth in appropriate environments. The High Energy Laser Joint Technology Office is pushing state-of-the-art, solid-state lasers for tactical applications. Finally, advances in cooperative beacons and advanced adaptive optics are enabling relay-mirror technology to emerge as a significant force enhancer for all high-energy laser systems. These are indeed exciting times, but it is a very tough job and the Air Force cannot expect to be totally successful. Future combat capabilities, as forecasted by these demonstrators and to the degree they are successful, will greatly benefit the US war fighter and provide unprecedented asymmetric war-fighting capabilities for the twenty-first century.

1. Dio Cassius, Roman History, vol. 2, Fragments of Books 12–35, trans. Earnest Cary and Herbert B. Foster (Cambridge, MA: Loeb Classical Library, Harvard University Press, 1914).

2. For an excellent account of the historical development of high-energy lasers, see Robert W. Duffner, Airborne Laser: Bullets of Light (New York: Plenum Trade, 1997).

3. USAF Scientific Advisory Board, New World Vistas: Air and Space Power for the 21st Century: Directed Energy Volume (Washington, DC: USAF Scientific Advisory Board, 1995).

4. “Directed Energy Worth Analysis and Vehicle Evaluation (DE-WAVE),” conducted by Lockheed Martin Aeronautics Corporation; “Tactical High Energy Laser Fighter (TAC-HELF) Study,” by Boeing Corporation; and “High-Energy Laser Weapons Systems Applications,” the Defense Science Board’s June 2001 summer study, are three such analyses.

The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.